The present invention relates to light-emitting element drive circuits, and more particularly to a drive circuit suitable as a laser diode drive circuit.
A laser diode is a major component of an optical transmission system. The laser diode, which suffers poor yields due to manufacturing variations, accounts for a rather high percentage of the cost of the optical transmission system.
In a laser diode, it is difficult to maintain stable oscillation output because of fluctuations in characteristics based on manufacturing variations, changes in temperature characteristics of the laser diode, and changes over time (aging). Thus, to compensate for such variations, the laser diode typically has its drive current (forward current), If, controlled by an automatic power control (APC) circuit.
FIG. 3 is an electrical block circuit diagram of an automatic power control (APC) circuit. In FIG. 3, a laser diode 1 is supplied a drive current If that combines a bias current Ib output from a bias current drive circuit 2 and a light-emitting current Ip output from a data current drive circuit 3. That is, the bias current drive circuit 2 outputs the bias current Ib having an identical value to the current (threshold current Ith) where the laser diode 1 starts to emit light. The data current drive circuit 3 outputs the light-emitting current Ip based on data Tx.
Thus, the laser diode 1 emits light in response to the light-emitting current Ip output from the data current drive circuit 3, that is, in response to the data Tx. A monitoring photodiode 4 receives the light of the laser diode 1, and outputs a monitor current Im that corresponds to the optical output Po received. The monitor current Im of the monitoring photodiode 4 is output to a shunt register R. A feedback circuit 10 receives a monitor voltage that is in proportion to the monitor current Im associated with the shunt register R, and compares the resulting monitor voltage with an initial setup value that is set by factory default, for example. In other words, as shown in FIG. 4, if characteristic A during initial setting of the laser diode 1 changes to characteristic B due to aging, temperature variations, and other factors of the laser diode 1, so that the optical output Po decreases relative to the drive current If, then the monitor current Im also decreases correspondingly. Thus, when the monitor current changes so that it is no longer the initial setup value, the feedback circuit 10 outputs a first control signal CS1 and a second control signal CS2 to the bias current drive circuit 2 and the data current drive circuit 3, respectively, so that the monitor current Im is the current at the time of initial setup
The bias current drive circuit 2 and data current drive circuit 3 adjust the bias current Ib and light-emitting current Ip, respectively, based on the first control signal CS1 and second control signal CS2. When the bias current Ib and light-emitting current Ip are adjusted, that is, the drive current If is adjusted, the optical output Po is controlled to the output Pos of the initial setting in the laser diode 1. That is, even if aging or temperature changes occur, the laser diode 1 is controlled so that the optical output Po is always maintained constant relative to the data Tx.
Meanwhile, it is important, from the standpoint of adjusting the bias current Ip and controlling the light emission of the laser diode 1, to measure the extinction ratio (ratio of optical output during lighting and optical output during extinction). That is, the laser diode 1 suffers changes in the threshold voltage Ith of the laser diode 1 due to aging, temperature variations, and so forth. More specifically, characteristic A during initial setting changes to characteristic B as the laser diode suffers changes in the threshold current Ith to Ith1 due to aging, temperature variations, and so forth, as shown in FIG. 4. Thus, it is important to measure the threshold current Ith and accurately control the bias current Ib and light-emitting current Ip based on the measured threshold current Ith.
However, with the aforedescribed automatic power control (APC) circuit, control cannot be achieved in consideration of changes in the threshold current Ith. That is, the feedback circuit 10 of the automatic power control circuit smoothes the monitor current Im to a direct current via a filter circuit within the circuit 10, thereby detecting a change in the optical output Po. As a result, changes in the threshold current Ith cannot be detected, so that the bias current Ib and light-emitting current Ip cannot be controlled in consideration of changes in the threshold current Ith.
The present invention is designed to solve the aforedescribed problem and has as its objective to provide a light-emitting element drive circuit capable of controlling the drive current of the light-emitting element in accordance with changes in the threshold current thereof.